The present invention relates broadly to an improved system for controlling the operation of a turbine power plant; and more particularly to an improved speed measurement system, for controlling the rotational speed of the turbine which provides rapid updating and high resoluting capability for direct digital input of turbine speed to a digital processor of the system.
In systems for controlling the operation of a turbine power plant, the rotational speed of the turbine shaft is considered in a control loop either as an end controlled or intermediate controlled system variable. Various systems for determining turbine speed have been developed over the years in order to provide an accurate speed signal for positioning the speed inlet valves to control the outlet of the turbine.
For example, an analog control system is described in U.S. Pat. No. 3,098,176, ELECTRIC LONG RANGE SPEED GOVERNOR, by M. A. Eggenberger, P. H. Troutman and J. F. Sauter, whereby a tachometer generator, which is connected to the turbine shaft generates a DC signal having a magnitude proportional to the actual speed of the turbine. In U.S. Pat. No. 3,097,488, TURBINE CONTROL SYSTEM, by M. A. Eggenberger, P. H. Troutman and E. C. Callan, there is described a system with a permanent magnet generator that is attached to the turbine shaft which generates an AC signal having a frequency proportional to actual turbine speeds. This AC signal is converted to a DC signal by saturable magnetic cores to provide a feedback voltage signal proportional to the frequency of the AC signal; and therefore, the speed of the turbine. The more recently developed turbine control systems, utilize a toothed wheel connected to the turbine shaft; and pulses generated by a reluctance pick-up adjacent to the toothed wheel are translated into a DC voltage which is utilized in an analog control circuit. A typical circuit for accomplishing the desired translation from a pulse frequency to a DC voltage is shown in U.S. Pat. No. 3,090,929, CONTROLLER CIRCUITRY WITH PULSE WIDTH MODULATOR by F. P. Thompson, assigned to the same assignee as this application. The frequency to voltage conversion technique described in the Thompson patent has operated satisfactorily in an electrohydraulic control system such as is described in an article by M. Burnbaum and E. G. Noyes presented to ASME-IEEE NATIONAL POWER CONFERENCE in Albany, N.Y., Sept. 19-23, 1965. In applying the frequency to voltage conversion technique to this type of system, the speed voltage may be applied directly to a control network of the general type described in U.S. Pat. No. 3,452,258, DIGITAL ANALOG FEEDBACK CONTROL SYSTEM EMPLOYING SOLID STATE DIGITAL POTENTIOMETER, by F. P Thompson, also assigned to the same assignee as this application.
With the advent of the turbine control systems of the digital-electrohydraulic type rendered the frequency to voltage conversion technique impractical. A system of this type is described in great detail in copending U.S. patent application Ser. No. 408,972, which is a continuation of Ser. No. 247,877, now abandoned, which is a continuation-in-part of Ser. No. 247,440, now abandoned, and entitled GENERAL SYSTEM AND METHOD FOR STARTING, SYNCHRONIZING AND OPERATING A STEAM TURBINE WITH DIGITAL COMPUTER CONTROL, all filed by Theodore C. Giras and Robert Uram and assigned to the present assignee; said original application being filed on Apr. 24, 1972. This is a consequence of the fact that digital equipment, without an analog or digital converter, operates only in response to digital input signals. For example, the central processing unit of a digital computer continuously performs digital routines under the control of programmed instructions. However, since a computer can perform only one operation at a time, externally generated data can only be accepted by the computer by interrupting the routine in process or by waiting until the routine which is running has been completed. Determinations of this nature are made by the executive program which establishes priorities for the various routines including the input routines.
In real time control, various systems status signals are generated independently in the computer cycle time. System conditions which can be expressed in terms of yes or no, or on or off, can be monitored by switches or relays which by their very nature generates signals in binary form. The states of the variable being monitored is stored by the condition of the switch or relay until the central processing unit of the computer is ready to accept the stored data. Such inputs are known as contact inputs. Numerous schemes for multiplexing and paralleling contact inputs have been developed to improve the efficiency of the computer system. Where analog functions are input to the computer, they must be transformed into digital signals before they can be accepted. Many types of analog to digital converters have been developed to form this transformation. Such transformation may typically be performed by converting the DC voltage into pulses having a frequency which is a function of the analog voltage level in a voltage to frequency converter. The pulses so generated are indicated in a digital counter and the resultant signal is fed into the central processing unit of the computer. Such conversion takes time, and as frequent sampling of the variable being monitored is central to proper dynamic control of the system, the converter may be engaged for a considerable period of its operating time merely monitoring a single analog signal. Economic considerations dictate the number of analog to digital converters that can be provided in a system.
In copending U.S. patent application Ser. No. 412,513, filed by John F. Reuther, TURBINE SPEED CONTROLLING VALVE OPERATION, filed Nov. 2, 1973 and assigned to the same assignee as this application, there is disclosed an improved system for determining the speed of rotation of the output shaft of a turbine where pulses generated by a displacement pulse generator connected to the turbine shaft are digitally transformed into signals directly usable by a computer in digital input equipment.
In this system, displacement pulses are generated at a rate proportional to the rotational speed of the turbine. Time pulses are generated at a predetermined rate. The rate of the timing pulses is substantially greater than the generated rate of the displacement pulses at the upper range of turbine speed. A number of displacement pulses are counted during a predetermined time interval; and the number of timing pulses are counted during the generation of a predetermined number of the displacement pulses. A signal corresponding to the counted number of the displacement pulses is generated as well as a signal corresponding to the counted number of the timing pulses. The timing pulses are utilized to control the steam inlet valves when the speed of the turbine is above a predetermined speed; and the displacement pulse count is used to operate the steam inlet valves when the speed of the turbine is below a predetermined speed. In other words, the speed is determined either as a function of the time interval required to accumulate a preselected count of the displacement pulses when the turbine is operating above a predetermined speed; or the speed is determined as a function of the number of displacement pulses accumulated in a predetermined time interval when the turbine is operating below a predetermined speed. in order to avoid losing time pulses during the interval required for inputting the timing pulse count into the computer, both the counter for the timing pulses and the displacement pulses are momentarily inhibited when the displacement count becomes equal to the preselected displacement count. Resumption of counting by both the displacement pulse counter and the timing pulse counter is resumed in a synchronous manner with the occurrence of the displacement pulses to insure accuracy.
In the copending Reuther application, the speed signal, calculated as a function of the time required to accumulate a predetermined count of displacement pulses is referred to as the fine speed signal because it has a resolution of approximately plus or minus 1 rpm at the synchronous speed of the turbine. The speed signal, which is calculated as a function of the number of displacement pulses accumulated in a predetermined time, is referred to as the coarse speed signal because it has a resolution of approximately plus or minus 5 rpm's at synchronous speed. However, when it is considered that the speed signal must be derived over the full operating range of the turbine, it is apparent that at the lower speeds, during start-up or shut-down, the fine speed calculation is not practical. This is due to the length of the time intervals required to accumulate the predetermined number of displacement pulses, such as 720, for example. At the lower rpm's the timing pulse count would be excessive requiring a counter with many more digits. Although the system of Reuther is accurate and reliable over the full range of turbine operating speeds, it is necessary to calculate both the coarse and fine speed signals; and then test the coarse speed signal to see if it is below or above a switch-over speed. This, in effect is a dual system. In view of the above, it is desirable to provide a system which is accurate and reliable over the full range of turbine operating speeds without the necessity of a dual system. Further, it is desirable to be able to provide a speed determining system that may be directly connected to the inputs of a digital computing apparatus without the necessity of logic circuitry for selecting one or the other of two calculated speed signals; and to provide a speed determining system where the direct count of pulses in a pulse train during predetermined intervals corresponds to the binary number corresponding to the rotational speed without further calculation.